The term “link” is often used to refer to the connection between two devices on a computer network. The link may be a physical medium, such as a copper wire, a coaxial cable, any of a host of different fiber optic lines or a wireless connection. In addition, network devices may define “virtual” or “logical” links, and map the virtual links to the physical links. As networks grow in size and complexity, the traffic on any given link may approach a maximum bandwidth capacity for the link, thereby leading to congestion and loss.
Multi-protocol Label Switching (MPLS) is a mechanism used to engineer traffic patterns within Internet Protocol (IP) networks. By utilizing MPLS, a source device can request a path through a network, i.e., a Label Switched Path (LSP). An LSP defines a distinct path through the network to carry packets from the source device to a destination device. A short label associated with a particular LSP is affixed to packets that travel through the network via the LSP. Routers along the path cooperatively perform MPLS operations to forward the MPLS packets along the established path. LSPs may be used for a variety of traffic engineering purposes including bandwidth management and quality of service (QoS).
A variety of protocols exist for establishing LSPs. For example, one such protocol is the label distribution protocol (LDP). Procedures for LDP by which label switching routers (LSRs) distribute labels to support MPLS forwarding along normally routed paths are described in L. Anderson, “LDP Specification,” RFC 3036, Internet Engineering Task Force (IETF), January 2001, the entire contents of which are incorporated by reference herein. Another type of protocol is a resource reservation protocol, such as the Resource ReserVation Protocol with Traffic Engineering extensions (RSVP-TE). RSVP-TE uses constraint information, such as bandwidth availability, to compute and establish LSPs within a network. RSVP-TE may use bandwidth availability information accumulated by a link-state interior routing protocol, such as the Intermediate System-Intermediate System (IS-IS) protocol or the Open Shortest Path First (OSPF) protocol. RSVP-TE establishes LSPs that follow a single path from an ingress device to an egress device, and all network traffic sent on the LSP must follow exactly that single path. The use of RSVP-TE, including extensions to establish LSPs in MPLS, are described in D. Awduche, “RSVP-TE: Extensions to RSVP for LSP Tunnels,” RFC 3209, IETF, December 2001, the entire contents of which are incorporated by reference herein.
In some cases, RSVP-TE can be used to establish a point-to-multipoint (P2MP) LSP that can be used for sending traffic across a network from a single ingress to multiple egress routers. The use of RSVP-TE for establishing P2MP LSPs is described in R. Aggarwal, “Extensions to RSVP-TE for P2MP TE LSPs,” RFC 4875, May 2007, the entire contents of which are incorporated by reference herein. A P2MP LSP is comprised of multiple source-to-leaf (S2L) sub-LSPs. These S2L sub-LSPs are set up between the ingress and egress LSRs and are appropriately combined by the branch LSRs using RSVP semantics to result in a P2MP TE LSP.